1. Field of the Invention
The present invention relates to calculating the geometric position of a mobile user based on signals from the orbiting GPS satellites. More particularly, the present invention relates to overcoming the inherent inaccuracies found in such positioning systems, even those using differential methods as applied to the GPS system.
2. Background Information
The Global Positioning System (GPS) has been known and used for a number of years and only a cursory review is included herein. The GPS system includes a number of orbiting satellites each of which contains data describing the satellite orbits. Time and phase differences among the satellite signals reaching a receiver are used to determine the position of the receiver. L-band signals and differential techniques are also available to help correct for inaccuracies due to the atmosphere. For example U.S. Pat. No. 5,323,322 (""322) to Mueller et al. describes a compensating differential GPS invention using fixed receivers at a known locations. That system provides for a central processing facility that accepts information from the satellites and the fixed receivers and broadcasts correction information to a user. The user can calculate its position by receiving the GPS satellite signals and using correction information from the central processing facility to determine its true geographical position within some tolerance. With the differential GPS, accuracies approaching one meter may be realized. However, as the distance from the fixed receivers and/or the central processing facility increases inaccuracies also increase.
U.S. Pat. No. 5,043,736 (""736) to Darnell et al. describes an earlier system using the cellular system for a base station to communicate with remote units where the base station provides the calculations that it then sends to the remote units.
In the present state of the art, a commercially available global positioning system (GPS) receiver is used to acquire signals from multiple GPS satellites and, from the data so obtained, calculate the geometric position of the user. Such a commercial system is inherently limited because of the accuracy of the signals provided by the GPS satellites, variable atmospheric conditions, and inherent system errors. Accordingly, the positional accuracy of the commercial GPS system is on the order of one meter. The present invention is directed to improving the positional accuracy of mobile GPS users
The foregoing objects are met in a differential positioning system for one or more mobile GPS users. The system includes a central processing facility and a plurality of reference stations with accurate geographic locations known to the central facility. The reference stations are arranged to receive GPS signals and communicate those received GPS signals to the central processing facility. The central processing facility then calculates the difference between the known accurate locations of the reference stations and their locations as derived from the GPS signals The central processing facility then generates correction data for the area associated with the locations of the reference units. Any mobile user in the region receives its GPS signals and sends those signals or its calculated location to the central processing facility. The central processing facility responds by sending to the mobile user its specific corrected location.
The system can be used to generate a mathematical, topographical model over the entire range covered by the reference stations and basically any mobile user within that area can be precisely positioned. In addition, the mobile user can generate a local error or offset from the last corrected position received and compare it to its received GPS signals and then use that offset in calculating its own position.
The above limitations are addressed and precise positioning of one or more mobile users is provided by a differential GPS system where a central processing facility communicates with the mobile user. The mobile user receives GPS signals that are available to it and sends measurements from the GPS signals to the central processing facility.
The central processing facility also communicates with a number of fixed stations, where the geographical locations of the stations are known. The stations also receive GPS signals, although not necessarily from the same satellites, and not necessarily from the same satellites communicating to the mobile users. The reference stations send measurement data derived from the satellite signals to the central processing facility. The central processing facility calculates the differences between the received measurement data and the known locations of the satellites and the references stations. An error model is calculated by the central processing facility. The mobile users receive GPS signal and derive measurement data therefrom that are sent to the central processing facility. The central processing facility using the error model determines a precise location of the mobile users which is then sent to the mobile users.
In an embodiment, the difference information may be used to generate a topographical model into which mobile users are placed, and therefrom correct positions of the mobile users may be calculated.
In other preferred embodiments, the mobile users might use the GPS signals to calculate their positions, and the mobile users may use the last position from the central processing facility to generate a local error that may be used to correct its calculated position.
Depending upon the temporal changes in the measurements and/or other factors that cause errors and the preciseness required by the mobile users, the central processing facility may update the mobile users positions on a regular time basis. Updating the positions more often will produce a more accurate position.
In an embodiment, the position of the mobile users may be presented on a map, as geographical coordinates, or as navigational guidance, e.g. including directional headings (northeast) or fifteen degrees, etc. The history of the positions may be stored and displayed.
Other features, objects and advantages will be apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings in which: